1. Field of the Invention
The present invention relates to a scanner for scanning an object to be scanned utilizing a rotational movement of a rotatably movable member mounted on a predetermined rotation axis on a rotatably movable basis.
2. Description of the Related Art
Hitherto, the above-mentioned type of scanners has been used in extensive fields. For example, according to a geophysical sensor working in a satellite in low orbital flight, an image of the earth is produced by means of joining together partial images of the earth's surface by several thousands of pieces each coming once into the visual field. At that time, photography is performed in such a manner that a scan mirror, which is rotatably movable on a rotation axis in an orbit direction, sits in front of a camera, the earth surfaces are scanned by changing the scan mirror in a direction little by little in its angle with respect to the earth, and the the earth surfaces are sub-scanned by moving the scan mirror on the orbit of the satellite around the earth. Alternatively, as an example of another type of scanner other than the above-mentioned type of scanners, according to a geophysical sensor working in a stationary satellite, an image of the earth is produced by means of joining together partial images of the earth's surface by several pieces to several tens of pieces each coming once into the visual field. At that time, photography is performed on one by one image in such a manner that a scan mirror, which is rotatably movable on two rotation axes perpendicularly intersecting, sits in front of a camera, the earth surfaces are scanned by changing the scan mirror in a direction little by little in its angle with respect to the earth.
FIG. 12 is a schematic perspective view of a scan mirror scanner according to the earlier technology.
A scan mirror scanner 100 comprises an elliptical scan mirror 101, a drive motor 102 for rotatably moving the scan mirror 101 in arrow A-A' directions through a support axis 106, and a drive motor 103 for rotatably moving the scan mirror 101 in arrow B-B' directions. The drive motors 102 and 103 are provided with rotary encoders 104 and 105, respectively. The rotary encoders 104 and 105 serve to detect rotary angles in the arrow A-A' directions and the arrow B-B' directions, respectively.
The scanner 100 is loaded onto an artificial satellite. The scan mirror 101 serves to reflect individual portions of the earth, and the earth surface is scanned on a two-dimensional basis through a rotatable movement of the scan mirror 101. In this manner, photography of images of the individual portions of the earth surface is performed by a camera (not illustrated) which is set up at an image position.
According to the above-mentioned satellite in low orbital flight, while it is possible to obtain high resolution (e.g.; a resolution expressed by a unit of a radius of 50 meters of the earth surface), it is impossible to take a picture until the satellite revolves one turn to reach a place of interest in photography and thus it lacks an operability on a real-time basis such that the state of affairs can be immediately photographed. On the other hand, according to the above-mentioned stationary satellite, simply scanning of the scan lines makes it possible to photograph anywhere within the scannable range and thus it is excellent in an operability on a real-time basis. However, it is simple to obtain a low resolution (e.g. a resolution expressed by a unit of a radius of 1 kilometer of the earth surface), and thus it is simple to be utilized for obtaining weather information, for instance, concerning a large cloud as a photographic object. Consequently, it is impossible to take a picture as to a fine situation such as the state of affairs.
It is necessary for the scanner 100 to have in addition to the scan mirror 101 a support axis 106 for supporting the drive motor 103 and the rotary encoder 105. The support axis 106 is strong, big and heavy. Further, it is necessary for the drive motor 102 to support and rotatably move in addition to the scan mirror 101 the drive motor 103, the rotary encoder 105, and the support axis 106 as well. Consequently, it is necessary for the drive motor 102 to use a large sized and heavy drive motor capable of generating a large drive force. Thus, this type of scanning mirror scanner is obliged to be large and heavy in its entirety, and it is not suitable in structure for a sensor which is loaded on an artificial satellite limited in weight on board.
According to the scan mirror scanner 100, the rotary encoders 104 and 105 detect rotary angles of the scan mirror 101 in the arrow A-A' directions and the arrow B-B' directions, respectively. In general, a rotary encoder adopts a scheme such that a slit plate is provided between a light-emitting device and a light receiving element or a photo detector, and photo pulses, which are generated through a rotation of the slit plate, are counted. Accordingly, in order to enhance a resolution, there is a need to shorten slit pitches. Assuming that the radius of a slit is of 5 centimeters to obtain a resolution of 1 kilometer from 36000 kilometers in the sky, however, there is a need to provide an about 1.4 .mu.m of slit width. This will be a limit in slit pitch in view of the problem on a practical precision. Further, while the rotary encoder is provided with slits throughout the slit plate 360.degree. around, the rotary angle of the scan mirror is only a part of the slit plate 360.degree. around Therefore, such an aspect that an angle can be measured throughout the slit plate 360.degree. around is not completely effectively utilized.